Since prostaglandins (hereinafter referred to as “PG(s)”) exhibit various important physiological actions in trace amounts, the syntheses of natural PGs and a great number of their derivatives as well as their biological activities have been investigated with the intention of practical use as medicines, and have been reported in many types of literature.
Particularly, while various central nervous actions of PGs have been reported, their intracerebral contents, biosynthesis, metabolic pathways, intracerebral localizations and changes with growth or aging have been revealed. Interest has been maintained on their relations with sleep or arousal by PGs. Particularly, PGD2 has been known as an intracerebral humoral factor that regulates the occurrence or maintenance of sleep; and it became clear that the sleep induced by PGD2 in monkeys is indistinguishable from their spontaneous natural sleep in brain wave or behavior (Proc. Natl. Acad. Sci. USA, Vol. 85, pp. 4082-4086, 1988). PGD2 was, therefore, expected to be a new compound possessing sleep-inducing action. However, PGD2 derivatives including PGD2 have not been put in practical use because of problems on their brain penetration and stability.
WO99/61419 discloses a compound having an ethynylene group at its α-chain. This compound is difficult to crystallize and presents the problem of formulation.